Online ticketing marketplace StubHub is aiming to go public by late summer, people close to the company said, setting up another sizable internet initial public offering after Reddit’s IPO last month. And as StubHub has more than $2 billion in debt, many times its estimated profits, the company will test investors’ appetite for companies with strained balance sheets.Further complicating matters, StubHub has ambitious hopes for the valuation it would achieve—preferably in line with the $16.5 bill...